For example if you have many images at your Forum or website, then you could "send" this image to a friend.
Well, you actually do not send the image itself via E-mail, but you just send a specific "Link-URL" to your friend.
The friend then needs to register at the website, in order to "view" the image at the website via this specific Link-URL.
It is exciting because every user will WANT to register in order to see the image (picture) which his friend wants to see him.
human psychology works like this:
if a friend sends you an image, then you want to see this image.... and therefore you will register at the website.... and probably also start to use this website.
Think about Megaupload.com , they had the same viral-marketing strategy:
You could NOT send a big file via E-mail to your friend. So what to do ?
You uploaded this big file to Megaupload.
Then you (or the system) just send a specific Link-URL to your friend.
Then your friend could go to Megaupload and download this big file via access to a specific Link-URL.
It is a very old, but free and viral internet-marketing-concept.
Think about sending "Digital Postcards".